Reject for inverse filtering in Ruby - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this Ruby code using reject?

Consider the following Ruby code that uses reject to filter an array. What will be printed?

Ruby

numbers = [1, 2, 3, 4, 5]
result = numbers.reject { |n| n.even? }
puts result.inspect

A [2, 4]

B [1, 3, 5]

C [1, 2, 3, 4, 5]

D []

💡 Hint

Remember, reject removes elements for which the block returns true.

The reject method removes elements where the block returns true. Here, it removes even numbers, so only odd numbers remain.

❓ Predict Output

intermediate

What does this code output when using reject with a hash?

Given this Ruby code using reject on a hash, what is the output?

Ruby

scores = { alice: 90, bob: 75, carol: 85 }
filtered = scores.reject { |name, score| score < 80 }
puts filtered.inspect

A {:bob=>75}

B {}

C {:alice=>90, :bob=>75, :carol=>85}

D {:alice=>90, :carol=>85}

💡 Hint

Reject removes pairs where the block returns true.

The block returns true for scores less than 80, so :bob is removed. The remaining pairs are alice and carol.

🔧 Debug

advanced

Why does this reject code raise an error?

This Ruby code raises an error. What is the cause?

Ruby

arr = [1, 2, 3]
result = arr.reject do |x|
  x > 1
end
puts result.inspect

A No error; code runs and outputs [1]

B SyntaxError due to missing 'end' keyword

C TypeError because reject expects a symbol argument

D RuntimeError because block returns nil

💡 Hint

Check if the block syntax is correct and what reject expects.

The code is valid. reject removes elements where block returns true (x > 1), so it removes 2 and 3, leaving [1].

❓ Predict Output

advanced

What is the output of reject with a complex condition?

What does this Ruby code print?

Ruby

words = ['apple', 'banana', 'cherry', 'date']
result = words.reject { |w| w.length <= 5 }
puts result.inspect

A ["banana", "cherry"]

B ["apple", "date"]

C ["apple", "banana", "cherry", "date"]

D []

💡 Hint

Reject removes words with length less than or equal to 5.

Words with length <= 5 are 'apple' (5) and 'date' (4), so they are removed. Remaining are 'banana' and 'cherry'.

❓ Predict Output

expert

What is the output of this nested reject code?

Analyze this Ruby code with nested reject calls. What is printed?

Ruby

data = { a: [1, 2, 3], b: [4, 5, 6], c: [7, 8, 9] }
filtered = data.reject do |key, arr|
  arr.reject { |x| x.even? }.empty?
end
puts filtered.inspect

A }]9 ,8 ,7[>=c: ,]6 ,5 ,4[>=b: ,]3 ,2 ,1[>=a:{

B :a=>[1, 2, 3], :b=>[4, 5, 6], :c=>[7, 8, 9]}

C {:a=>[1, 2, 3], :b=>[4, 5, 6], :c=>[7, 8, 9]}

D {}

💡 Hint

Check what the inner reject returns and when the outer reject removes a key.

Inner reject removes even numbers. If the resulting array is empty, outer reject removes that key. All arrays have odd numbers, so none become empty after inner reject. So no keys are removed, filtered equals original data.